The present invention relates, generally, to laser devices, and more particularly, to an improved laser device employing diode laser bars as the pumping means and a YAG rod as the gain medium.
The concept of pumping a neodymium (Nd) doped Yitrium Aluminum Garnett (YAG), commonly referred to as a Nd:YAG Laser with an array of semiconductor laser diodes is hardly new; but the use of laser diodes to pump a high energy gain medium laser has not been widely considered in the past. There have been many papers published and U.S. patents issued describing low power diode pumped lasers, e.g., U.S. Pat. No. 3,624,545, Semiconductor Pumped Laser, which teaches linear arrays of individual laser diodes. This approach can be scaled up to drive Nd:YAG Lasers at modest power levels, but the applications have been limited to those requiring continuous wave (CW) pumping, primarily because of the limitations in peak power and power density imposed by the use of single laser diodes, and in all cases it has been necessary to cool the linear arrays of individual laser diodes used to pump the laser medium. Until now, the design of diode pumps has been based on individual diode chips because the uniformity of wafers has been poor, both with respect to diode performance and output wave length. Continuous wave (CW) pumping, primarily because of the limitation in peak power and power density imposed by the use of single diodes which simply cannot be arranged physically in sufficient density around the medium to be pumped, has been the standard method of laser pumping. Of course, to run a diode laser CW required the laser to be cooled.
The laser diode bar is a bar of material cleaved from a wafer which contains many individual diode contact stripes, each of which is equivalent to a single diode. The overall improvement in diode packing density that is possible using a bar array approach is between forty and two hundred times over single diode pump arrays.
All lasers include a laser medium having a near 100% feed-back mirror at one end and a partial feed-back mirror at the opposite end so as to form a cavity. The mirrors define an optical access extending through the cavity in a direction perpendicular to both mirrors. In the case of the solid state laser, as here, the crystal rod itself may be polished so the polished surfaces reflect the beam many times before emerging. The lasing medium, in this case a YAG rod, forms the laser cavity and the atoms in the lasing medium have quantum energy levels which define allowable rotational, vibrational and electronic energy states for the atoms. In the absence of external stimulation, most of the atoms in the lasing medium subsist in the lower energy status E.sub.1. As a result of random energy transfer effects, however, a small fraction of the atom population will normally be in a higher energy level E.sub.2 at any given moment. A beam of photons interacts with free atoms in the ground state E.sub.1 or the excited state E.sub.2 in many ways. Three of these ways are of particular interest. The photon interacts with an atom in the ground state E.sub.1, the atom absorbs the photon and is left in the excited state E.sub.2. This process is called induced or stimulated absorption. An atom in the excited state E.sub.2 can drop back or decay by either spontaneous emission or induced or stimulated emission. In spontaneous decay the radiation is emitted in all directions or is noncoherent. In induced emission, the photons induce the atoms in state E.sub.2 to decay by emitting photons traveling in the same direction as the incident photon. The result is a unidirectional coherent beam. For every incident photon we have two outgoing photons and this is the reason why light amplification takes place. The basic requirement of a laser is to have predominently induced transitions so that the radiation is coherent and phase related to the incident radiation. Induced transitions are achieved by first causing population inversion, which occurs when there are more atoms in the higher energy state E.sub.2 than the lower energy state E.sub.1. During discharge of the laser some of the photons emitted by the first atoms to decay are emitted in a direction parallel to the optical axis of the laser cavity. Since one end of the cavity is fully mirrored and the other end is partially mirrored, these photons, on the average, undergo multiple reflections between the two mirrored surfaces before they finally pass through the partial feedback mirror at the emission end of the cavity. During the course of these multiple internal reflections within the laser cavity, these photons cause stimulated or induced emission of additional photons by emitting photons traveling in the direction of the incident photon, i.e., parallel to the optical axis of the laser. The photons emitted by induced or stimulated emission also undergo multiple reflections between the mirrored surfaces and continue the process to form the now well-known beam of highly coherent, monochromatic laser radiation.
This process of raising the atoms to an excited energy state to establish a population inversion is known in the art as pumping the laser. Many different methods and techniques are used to pump different kinds of lasers. The Nd:YAG laser in the instant case is pumped by another laser, a semiconductor or diode laser. The diode laser functions just as any other laser, as described above. Specifically, typical semiconductor lasers are made by defusing another metal into N-type gallium arsenide to form the p-n junction. While some light is emitted in directions away from the junction, some light will move along the plane of the junction and will trigger off additional light signals by the process described above as induced or stimulated emission. The important fact is that the stimulated signal will be an exact replica of the triggering signal and so will add to it. Thus the initial signal moving along the junction will be multiplied many times before reaching the end of the junction. The front and back ends of the junction function as the mirrors discussed above. Some light is reflected back and forth between the front and back ends of the junction further amplifying between each bounce, and a coherent light output emerges from the edge of the junction and produces laser action.
The diode arrays, whether linear arrays of individual diodes as in the old art or arrays of diode bars as in this invention, are generally positioned similarly around the laser rod sleeve as shown in FIGS. 5, 6, and 7. The laser diode arrays emit a fan-shaped beam approximately 30.degree. by 60.degree. full angle with the smaller dimension oriented parallel to the laser rod axis. The beam incident on the rod sleeve surface is focused by the inside curvature so that the pump light is concentrated near the rod axis.
The limitations in peak power and power density of single diode pump arrays, as used in the prior art, are such that the array must be driven by a continuous wave. The slope efficiency of a laser diode is usually much higher than its absolute efficiency because it is operated above threshhold; hence, much of the input power is consumed maintaining threshhold. Hence, internal quantum efficiency is greatly increased by driving the diode in a series of short pulses rather than the long pulses associated with a continuous wave. Thermal affects on diode performance are also significantly reduced by driving the diode with very high frequency pulses for short periods. Continuous wave form requires cooling of the diode, as in all prior art CW driven semicontinuously pumped arrays because heating increases or shifts the wavelength of the diode. The Nd:YAG rod has a desired absorption spectrum which needs to be matched by the pump wavelength for efficient operation. Pulsing the diodes minimizes the heating and resulting shift in diode wavelength. Furthermore, a continuously pumped semiconductor laser array has a short life.
It is an object of this invention to produce a semiconductor laser array for pumping another laser cavity which has a continuous output without cooling the semiconductor pump array and yet functions for a very long lifetime and is very small in size. Another object of this invention is to produce an improved Nd:YAG laser by maintaining the spectral match between the gain medium and the pump. It is a further object to accomplish the foregoing objectives and produce a diode pump array with a high power and high power density output specifically for pumping another high powered output laser cavity.